1. Select the field or habitat to be sampled for

damage.

2. Assign the field or habitat a unique identification

number or name.

3. Estimate the total area of the field or habitat to be

sampled by walking the perimeter of the field or

habitat and logging your position continuously

using a hand-held GPS unit. Be sure to log your

position at an appropriate interval (i.e., every meter

or less) to ensure the perimeter is accurately

represented. Then, convert the information to a

Geographic Information System (GIS) compatible

file and use the GIS software to calculate the area.

Alternatively, Google Earth or high-resolution

Tech Note

March 2018

Using GPS Measurements 

to Estimate Damage

Global 

positioning 

systems (GPS) 

can be used to 

calculate highly 

accurate 

damage 

assessments .

4.

5.

Wildlife Services

Protecting People 

Protecting Agriculture 

Protecting Wildlife Part of the WS Damage Assessment Series

digital images may be used to trace the field 

perimeter and calculate the area using GIS 

software.

Locate individual patches of damage within the 

field or habitat's perimeter by systematically 

walking the site on parallel transects, typically 

20 to 30 meters (65 to 100 feet) apart. This 

transect spacing should allow the observer to 

detect all damaged patches within the site. If 

not, modify the spacing between transects to 

ensure damage can be seen. Note: Navigation 

and mapping functions in the GPS unit can be 

used to maintain parallel transects over uneven 

terrain.

When a patch of damage is encountered, walk 

its perimeter using the GPS unit to log your 

position continuously at an appropriate interval

(i.e., every meter or less).

This Tech Note provides a practical, in-field, global 

positioning system (GPS) measurement method for

gathering highly accurate (i.e., near-exact) damage 

estimates for resources with continuous ground 

cover. Example resources that are appropriate for 

this method include sensitive or endangered plant 

communities, pastures, and high-value crops that 

completely cover the ground (i.e., crop rows are

missing or indistinguishable). 

The method requires the use of a high-precision GPS 

unit that achieves sub-meter accuracy through 

differential correction (i.e., using real-time 

differential GPS or post-processing software).

Other, less labor-intensive measurement methods 

are available, but may not be as precise. 

Gathering Damage Polygons Using GPS
area of each damage patch by its proportion 

damaged.

DAMAGE AMOUNT for each patch = PATCH 

AREA x PROPORTION DAMAGED

The total amount of damage for the field or 

habitat is calculated by adding together all of 

the DAMAGE AMOUNTS for all of the damage

patches.

TOTAL DAMAGE AMOUNT = sum of all 

DAMAGE AMOUNTS over all damage patches

To convert the TOTAL DAMAGE AMOUNT to the 

proportion of field or habitat that is damaged, 

divide the the TOTAL DAMAGE AMOUNT by the 

TOTAL AREA OF THE FIELD OR HABITAT.

7.

United States Department of Agriculture

Animal and Plant Health Inspection Service

The U.S. Department of Agriculture (USDA) is an equal opportunity provider and employer. 

6.

Additional Information

For more information on estimating

damage using a GPS, please contact:

Eric Tillman or Richard Engeman 

USDA-APHIS-Wildlife Services 

National Wildlife Research Center 

4101 LaPorte Avenue

Fort Collins, CO 80521

(970) 266–6000

PROPORTION DAMAGED = TOTAL DAMAGE AMOUNT

TOTAL FIELD/HABITAT AREA

3.

2.

1.

Enabling the GPS unit to make an audible sound 

when positions are logged can facilitate this 

task. If the GPS unit has a pause button, use it 

when stopping or taking extra time to negotiate 

difficult terrain. Stopping or lingering will cause 

the unit to collect too many GPS positions at one 

point.. Note: If too few GPS positions are logged, 

the resulting polygon may not accurately reflect 

the overall patch of damage.

In some cases, damage patches may include 

remnants of unaltered vegetation. To avoid overestimating the amount of damage, visually 

estimate the percentage/proportion of each 

patch that is damaged. If the GPS unit is able to 

use a data dictionary or electronic form to 

collect feature attributes, use this tool to record 

the percentage of disturbed vegetation as an 

attribute of each damage patch.

Export the damage data file(s) from the GPS unit 

to a GIS-compatible file. Depending on the type 

of GPS unit you may need to first process your 

data through unit-specific software and then 

export to a compatible format (i.e., shapefile). 

GIS software is then used to calculate the area 

of the damage patch. Damage patch area and 

attribute data can be exported to an Excel 

format where additional calculations can be 

made.

Calculating Total Damage

For the damage patches that included 

undisturbed vegetation, multiply the calculated